1. Field of the Invention
The invention relates to the field of liquid separation membranes, and more particularly to a method for making a liquid separation membrane.
2. Description of the Related Art
Due to excellent chemical and thermal stability, the liquid separation membrane of polyvinylidene fluoride has been widely used for treatment and reuse of industrial wastewater and domestic sewage as well as pretreatment of reverse osmosis process. In use, the liquid separation membrane of polyvinylidene fluoride is subjected to the impact of water flow (including cross-flow filtration, backwashing, etc.), air flow (including air scrubbing, aeration shaking, etc.), or a combination thereof. Thus, the liquid separation membrane requires a high strength for ensuring the normal operation, which is particularly prominent in a submerged membrane module adopted in the technical application process of a membrane bioreactor (MBR).
Conventionally, there are two methods for industrial preparation of the liquid separation membrane of polyvinylidene fluoride: one is an immersion precipitation method. The membrane prepared by the method has the advantages of high separation accuracy and good hydrophilicity, but the improvement of mechanical properties of the prepared membrane is usually restricted, and the strength of the liquid separation membrane prepared by the method is enhanced only through increasing the thickness of the membrane itself and a sponge layer thereof, however, the membrane permeability is usually reduced. The other is a thermally induced phase separation method. The membrane prepared by the method has the characteristic of better mechanical properties, but the filtering accuracy and the permeability cannot usually be balanced. According to the separation membrane formation principle, there is no a compact layer on the surface of the membrane prepared by the thermally induced phase separation method, the embedded pollutants are easily formed in the using process and difficult to remove, and the requirement on cleaning process is usually high. Therefore, to ensure that the liquid separation membrane has both high interception accuracy and permeability and good mechanical properties has became an important subject of research and development of the liquid separation membrane.